In a typical boiling-water reactor, fuel is provided in a number of fuel rods. The fuel itself is in the form of cylindrical pellets of enriched uranium. Enrichment is the proportion of fissionable U.sup.235 to the non-fissionable U.sup.238. These pellets are enclosed in a long cylindrical tube and sealed at both ends. The cylindrical tube with the enclosed fuel pellets is known as a "fuel rod." The rods are provided in the reactor in a number of discrete packages, which are called "fuel bundles."
Each bundle includes a plurality of rods held between an upper tie plate and a lower tie plate. The tie plates contain seats or apertures for positioning and holding the ends of the fuel rods. Additionally, the tie plates include apertures for permitting a flow of water therethrough in the interstices between the fuel rods.
Each fuel bundle is surrounded by a fuel channel. This channel, which is typically square in section, extends from the lower tie plate to the upper tie plate. The channel functions to confine water flow in between the tie plates and around the fuel rods.
Typically seven spacers are substantially evenly spaced along the length of the fuel bundle inside the fuel channel. The spacers act to further position the fuel rods along their longitudinal extends. An upper handle portion typically attached to the upper tie plate and a lower nose piece protruding downward from the lower tie plate define the top and bottom of the fuel bundle. The handle and nose piece function to permit ready insertion and removal of the fuel bundles during so-called "reactor outages."
Individual fuel rods in a bundle are disposed in a matrix, and are normally arrayed in rows and columns. Typically, some of the rows and columns in the matrix are occupied by tie rods. The tie rods are threaded fuel rods which engage the upper and lower tie plates to provide structural integrity to the fuel bundle. A typical fuel rod is approximately 160 inches in length.
In a reactor, a plurality of fuel bundles are positioned in the reactor core. Fuel bundles are positioned between a lower core plate and an overlying top guide. The fuel bundles are supported within the core of the reactor at the elevation of the core plate, and are held in vertical spaced apart relationship at the top guide.
Each fuel bundle in the reactor core is typically spaced apart from its neighboring fuel bundles. This spacing establishes a water filled volume in the reactor core known as the core bypass region. Water is maintained in this core bypass region by metering of a small amount of water through the fuel bundle nozzles.
The nuclear reaction is controlled by a number of control rods or blades. These are typically in a cruciform shape so that each control blade is adjacent to four fuel bundles. The control rods are inserted into and out of the core bypass region. These control rods contain neutron absorbers such that insertion of the control rods will locally slow or stop the reaction from being critical.
During operation of the reactor, water enters the fuel bundle through the lower tie plate. The water rises through the fuel bundles because of heating, and also, where used, from the action of one or more pumps in forcing circulation through the reactor. As the water rises through the fuel bundles and is increasingly heated, during normal operation, it eventually reaches its boiling point. Steam is formed from the boiling water, causing steam voids in the upper portion of the fuel bundle.
The water in a boiling-water reactor performs two functions. First, the water carries away heat from the reactor so that it can be converted to useful energy by, for example, a turbine. Second, the water acts as a moderator, i.e., it slows down the "fast" neutrons.
Neutrons in a nuclear reaction are present at a variety of energy levels. The neutrons are generally referred to as "fast" neutrons and "slow" (or "thermal") neutrons. Slowing down of the fast neutrons is desirable for at least two reasons. First, the slow neutrons are more reactive in the sense that they maintain the desired chain reaction involving the fission of U.sup.235 atoms. Second, slower neutrons are more easily captured by the control blades than the faster neutrons. Therefore, a moderator, in effect, increases the efficiency of the control blades.
As noted, water is a moderator of fast neutrons. However, as water is heated, it becomes less dense and less effective as a moderator. When the water becomes steam, its effectiveness as a moderator decreases drastically, and can be, for some purposes, treated as a negligible moderator.
In early fuel bundle designs, all lattice positions in the bundle were occupied by fuel rods. In these early designs, the only space for water in the interior of a fuel bundle was the space between the fuel rods and in the interstitial volume between discrete fuel bundles. Because the space between rods is typically filled with a mixture of water and steam, the moderating effectiveness of this space is less than space between bundles containing "solid" moderator. Accordingly, the most effective moderating water of the reactor was positioned between fuel bundles, i.e., in the core bypass region in the interstices between the bundles, exterior to the fuel channels.
In such earlier configurations, the interior fuel rods in any fuel bundle were a large distance away from the large volumes of "solid" moderating water. Because of this distance, the most interior positions in the fuel bundle had large ratios of fast to slow neutrons, and were, therefore, less efficient in maintaining those nuclear reactions requiring "slow" or "thermal neutrons". Accordingly, interior rods were typically more enriched to compensate for this lack of efficiency. Such increase in rod enrichment, however, is rather expensive. Therefore, it was previously decided to provide for additional moderating water in interior positions of a fuel bundle.
Initially, one or more fuel rods were replaced with a hollow rod (called a "water rod") of equal diameter for flowing water therethrough. The water rod communicated with the lower tie plate and extended through the upper tie plate. The water rod has its own confined water flow path and as a consequence is (like the bypass region) filled with water moderator.
A water rod has nuclear and thermal advantages over simply leaving a spaced unoccupied by a fuel rod. By providing a hollow rod, the subcooled water inside is prevented from mixing with the other heated water in the bundle, and is somewhat insulated. The water in the rod, therefore, does not boil as does other water in the bundle.
This scheme provided some advantages because of the additional moderator in the interior positions of a fuel bundle. Initially, the water rods were the same size as the fuel rods. Later, attempts were made to provide larger diameter water rods in the fuel bundles, these later water rods exceeding the size of the ordinary fuel rod. These attempts to provide larger water rods involved merely positioning a standard round cross-section pipe, or, in some cases, a square cross-section pipe, in interior positions of a fuel bundle so as to displace one or more fuel rods. No effective attempt was made, however, to depart from spaced-apart, standard round or square tubes, or to systematically analyze the effect of these shapes on reactor efficiency.
Since fuel rods when viewed in horizontal section are arranged in rows and columns, it is common to refer to each fuel bundle as occupying a "lattice position". When the water rods were expanded in size, they intruded from one fuel rod position into those fuel rod positions occupied by adjacent fuel rods.
As water rod design progressed, a configuration was provided in which a water rod had a circular cross-section with a diameter sufficiently large that it occupied more than one lattice position. In one such design, four lattice positions were sacrificed to accommodate a circular water rod. Water rods have also been developed which have a substantially square cross-sectional configuration, and occupy four or nine such lattice positions.